Development of a novel bicombinatorial screen will allow isolation of small molecule compounds (about 180 daltons) that interact specifically with individual intracellular proteins. By screening a combinatorial library of compounds against a combinatorial library of protein targets the chance of discovering specific compound-protein pairs will be maximized. Typically a compound must interact with a protein very strongly in order to be specific. This screen will take advantage of tethering technology, which uses a secondary disulfide tether between the small molecule and a cysteine on the protein to stabilize interacting complexes. This allows specific binding even when the interaction energy is low, so the likelihood of finding interacting molecules is dramatically increased, and the compound library can be relatively small. The compounds can be rapidly converted to fluorescent tags to study protein localization, or to affinity compounds to isolate the protein they recognize alone and in complexes. These compounds will also be useful as novel lead compounds for drug discovery. In contrast to conventional drug screens which focus on a single protein target, the bicombinatorial screen should suggest both which chemical moities are the most effective in binding proteins, and which chemical constituents confer anti-specificity. It may also be able to suggest some unexploited families of proteins that would also be suitable as drug targets.